One aspect of this invention applies to the completion phase of oil and gas wells, which requires the use of substantially solids-free completion and/or packer fluids. If solids are present in the fluid, they can cause serious damage to a producing formation by plugging the formation pore spaces or the perforations and channels provided to permit fluid flow between the formation and the well bore. Solids in a packer fluid will eventually precipitate on the packer, making it difficult to connect or disconnect tubing from the packer, thus resulting in a costly well work over. In order to maintain a solids-free fluid, the surface equipment, well bore, casing, and the like, must be cleaned prior to introduction of the solids-free packer or completion fluid to the well bore.
Environmental regulations governing the disposal of oil and gas well drilling fluids have become increasingly restrictive. Such regulations have curtailed the use of conventional oil base muds (“OBMs”), which are based on petroleum products such as diesel or mineral oils, and have lead to the development of synthetic base muds (“SBMs”). Synthetic base muds are safer to the health of workers and to the environment. SBMs are more biodegradable and more dispersible in seawater or brine, than traditional oil base muds; however, they are more difficult to remove from the metal surfaces of drilling equipment. As used throughout this disclosure and claims, “oil,” “petroleum,” etc. are inclusive of their synthetic equivalents used in well drilling operations, SBMs, etc.
Certain solvent blends have been developed in an effort to remove SBMs during fluid displacement and well clean up processes U.S. Pat. No. 5,678,631 (Salisbury et al 1997 and others). However, the residual contaminants are difficult to remove due to any entrained solids or metal surfaces being coated with a film of a “sticky” synthetic base fluid. In some wells, solids build up on the casing. Paraffin solids in some cases contribute to this solid build-up and can be difficult to remove.
Prior clean-up operations, typically done during the displacement process, made use of various surfactants and/or solvents for removing drilling muds and for cleaning drilling and well bore equipment. U.S. Pat. No. 4,453,598 (Singer, et al., Jun. 12, 1984); U.S. Pat. No. 4,474,240 (Oliver, et al., Oct. 2, 1984); U.S. Pat. No. 4,528,102 (Oliver, et al., Jul. 9, 1985); U.S. Pat. No. 4,588,445 (Oliver, et al., May 13, 1986); and U.S. Pat. No. 4,592,425 (Oliver, et al., Jun. 3, 1986) disclose processes for cleaning muds from drilling systems using a surface active agent/surfactant and an alcohol. The combinations of alcohols and surfactants disclosed have a number of disadvantages in that they are only surface active and do not destabilize the oil based mud emulsion in OBMs. Therefore, cleaning is inefficient since it requires the oil from the OBM or SBM to be physically adsorbed or emulsified into the cleaning solution. The combinations are also less effective in removing synthetic base muds from drilling equipment and well bores. Strictly solvents based or strictly water based formulations both suffer from this deficiency.
A second aspect of the acid based micro-emulsion central to the present invention is the cleaning of OBM and SBM drill cuttings. During the drilling process large amounts of drill cuttings (shavings) are produced and carried to the surface by the return of the OBM or SBM to the surface. These cuttings are coated with oily OBM or SBM emulsion and must be disposed of in an environmentally sound manner. Current environmental restrictions limit the operators to a narrow range of disposal options including, but not limited to, land farming, thermal desorption, enhanced bio-degradation, solidification and cuttings injection. Washing of the drill cuttings is environmentally acceptable but not viable since current washing chemicals have been unable to remove the majority of the oil from OBM cuttings.
Through the investigation of the acid based micro-emulsions of the present invention, it has been determined that such acid based micro-emulsions used in a water washing operation destabilize the OBMs and SBMs and their emulsions that coat the cuttings' surface and water wet the solids, which allows for ease of separation of the oil, water and drill cuttings. The oil then can be recycled back to operations, the water recycled for reuse and the cleaned solids made available for disposal in a nonrestrictive environmentally sound manner.
A third aspect of this invention relates to the cleaning of surface oil field equipment including but not limited to storage tanks, sand removal equipment, produced sand and dirt and refinery sludges. The production of petroleum hydrocarbon, after the drilling process is complete, also results in the production of sand, dirt, clay and solids of varying compositions. These solids are natural to the subsurface environment and generally are held in suspension in an oil external emulsion produced naturally as a part of the production process. These solids are undesirable and are removed by several methods known well to the industry. These methods include but are not limited to chemical emulsion breaking, mechanical separation, filtration, centrifugation, the use of hydrocyclones and gravity separation. Through the investigation of the acid based micro-emulsions of the present invention, it has been determined that this invention has benefit as an emulsion breaker, solids wetter, sand cleaner and tank cleaner in surface petroleum operations.
Micro-emulsions are cleaning compositions generally comprised of a continuous phase of at least one aqueous surfactant component and a dispersed phase of one or more water-immiscible components, such as oils, fatty alcohols and/or terpenes. It is known that systems comprising a surfactant, water and these water immiscible components can assume different phase structures. Three types of phases, which comprise surfactant and water, are generally recognized: the rod phase, the laminar phase and the spherical micelle phase.
In the spherical micelle phase, surfactant molecules align in spheres having a diameter approximately twice the molecular length. For anionic surfactants in common use, these structures are less than 10 nm in diameter. Systems exhibiting this phase structure are clear, have a viscosity similar to water and cannot suspend particles.
The rod phase can be considered as a spherical phase, which has been encouraged to grow along one dimension. It is known that this can be achieved by the addition of oils. Typically, the rods grow to very large dimensions resulting in highly viscous solutions. Although the viscosity of these systems is high, suspended particles will eventually phase separate.
The laminar phase is (believed to be) characterized by the presence of extensive bilayers of aligned surfactant molecules separated by water layers. These systems are generally of lower viscosity than the rod phase systems, can be opaque and can suspend particles.
When an oil or solvent is added to a surfactant-water system, the oil can remain in a separate phase or form part of a mixed phase. The so-called “micro-emulsions” are believed to be oil-in-water emulsions wherein the dispersed oil droplets are sufficiently small that a substantially visibly clear system results. These systems have a low viscosity and will not suspend particles, but differ from spherical micelles in that they exhibit low interfacial tensions in the presence of other oily materials such as oil based drilling fluids. It is believed that the low interfacial tension enables the micro-emulsions to spontaneously emulsify added oily materials, giving a particular cleaning benefit as compared with spherical micelles.
As will be appreciated, micro-emulsions have a similar overall composition to the rod micelle systems which can be obtained by adding oil to a spherical micelle system, but have a completely different phase structure and distinct physical properties. It is believed that in the micro-emulsions the dispersed oil phase is segregated into discrete spherical droplets each stabilized by a surfactant shell, whereas in the rod phase the oil phase is mixed with the surfactant to form a cylindrical mixed micelle structure.
Patents GB 2190681 (Colgate; 1987) and EP 316726 (Colgate; 1987) disclose systems which comprise both anionic and nonionic surfactants, together with a co-surfactant, a water-immiscible hydrocarbon such as an oily perfume and water. Surfactants may comprise solely anionic surfactants, although mixtures of anionics and nonionics are preferred. According to these texts (see page 5, lines 31 ff. of GB specification 2190681), the co-surfactant is essential in that in the absence of this component the surfactants and the hydrocarbon will form a non micro-emulsion phase structure. Suitable co-surfactants are said to include glycol ether solvents such as butyl carbitol, which is miscible with water, and butyl cellosolve, which is highly water soluble.
GB 2144763 (P&G; 1983) relates to micro-emulsion systems which contain magnesium salts. Examples demonstrate that aqueous liquid compositions can be prepared with anionic surfactants alone and with mixture of anionic and nonionic surfactants.
U.S. Pat. No. 4,511,488 (Penetone; 1985) relates to compositions which are described as clear, flowable compositions and which comprise 10-60 weight percent of d-limonene (a citrus oil), 10-30 weight percent surfactant, and 2-70 weight percent water, in the presence of a coupling agent such as a glycol ether solvent, in particular butyl carbitol. It has been found by experiment that these compositions are not stable and phase separate rapidly on standing.
The disclosures of the above-referenced publications and patents are hereby incorporated herein by reference.
From the above it can be seen that micro-emulsions generally comprise water, a surfactant mixture, an oil and a solvent. The surfactants are typically mixtures of anionic and nonionic surfactant. The oil is generally a perfume oil, such as d-limonene. The co-solvent is often referred to as a “coupling agent” and is generally a glycol ether.
No previous work is known to have resulted in stable acid based micro-emulsions suitable for cleaning operations such as the oil field treatments described herein.